stl vector를 활요한 수식 연산 관련해서 질문입니다.

class AssessSciCal : public SciCal
{
public:
	AssessSciCal():SciCal("")
	{
 
	}
 
	void create_assessment(std::string name, std::string results)
	{
		std::stringstream sstream;
		sstream << name << "_" << results << ".assess";
 
		std::ofstream fout;
		fout.open(sstream.str());
		fout.close();
	}
 
	void Assess01_02()
	{
		m_expressionList.clear();
		m_ElementList.clear();
		m_expressionList = std::string("3 * ( 2 + 1 ) =");
		ParseExpression(m_expressionList);
		NumberElement* elem1 = EvaluateExpression(); <- 문제가 되는 함수
		NumberElement* elem2 = new IntegerNumber(9);
 
		if (elem1->get_val().integer == elem2->get_val().integer)
			create_assessment("pa02_02", "PASS");
		else
			create_assessment("pa02_02", "FAIL");
	}
};

class SciCal
{
public:
	SciCal(std::string expression);//뒤에서 만들어주면 되겠구만
 
public:
	// You don't have to implement the void ParseExpression(unsigned int idx) function
	// void ParseExpression(unsigned int idx);
 
	// You should implement the  ParseExpression function. [Mandatory]
	void ParseExpression(std::string expr);
 
	// You may use PushNumber function. [Not Mandatory]
	void PushNumber(std::string number);
 
	// This function prints each element in m_ElementList [Mandatory]
	void PrintElements();
	// This function prints the elements from the given list [Mandatory]
	void PrintElements(std::vector<Element*>& list);
 
	// This function prints the elements from the given list [Mandatory]
	std::vector<Element*> TransformPostfix(std::vector<Element*> element_list);
 
	// This function prints the elements from the given list [Mandatory]
	NumberElement* EvaluateExpression();
 
 
 
	OperationElement* GetOperationElement(char op);
	std::vector<Element*> GetElements() { return m_ElementList; }
 
protected:
	// You don't have to store the expression in the m_expressionList
	// std::vector<std::string> m_expressionList;
 
	// m_expressionList stores the given string expression using m_expressionList variable.
	// You must use it. [Mandatory]
	std::string m_expressionList;
	// m_ElementList stores elements of the expression.
	// You must use it. [Mandatory]
	std::vector<Element*> m_ElementList;
 
	std::vector<Element*> m_Postfixelements;
	TheStack* m_stack;
};

// This function prints the elements from the given list [Mandatory]
NumberElement* SciCal::EvaluateExpression()
{
	std::vector<NumberElement*> stack;
 
	for (std::vector<Element*>::iterator iter = m_Postfixelements.begin(); <- 이 부분에서 시작주소를 받아오지 못하고 바로 for문을 탈출합니다. 
	iter != m_Postfixelements.end(); ++iter)
	{
		if ((*iter)->get_type() != Element::OP)
			stack.push_back((NumberElement*)*iter);
		else {
			NumberElement* elem1 = stack.back();
			stack.pop_back();
			NumberElement* elem2 = stack.back();
			stack.pop_back();
 
			OperationElement* opelem = (OperationElement*)(*iter);
			NumberElement* evalelem = opelem->Evaluate(elem2, elem1);
 
			stack.push_back(evalelem);
		}
	}
	return stack.back(); <- 그래서 이 부분에서 제대로 된 반환을 하지 못하고 프로그램이 경고창을 띄우면서 종료됩니다. 
}